1. Field of the Invention
The present disclosure relates to radio frequency identification (RFID) systems, and more particularly to systems and methods of enhancing range in RFID systems.
2. Description of the Related Art
FIG. 1 illustrates a conventional RFID system 100. The RFID system 100 typically includes an RFID reader 110 and one or more RFID tags 120 (only one illustrated) that wirelessly communicate with the RFID reader 110.
Typically, the RFID tag 120 includes an antenna 122, a front end 124, and a signal processing/memory block 126. The RFID tag 120 may be passive, deriving power from an interrogation radio frequency (RF) signal sent by the RFID reader 110. In particular, the front end 124 may include rectifying circuitry for converting the AC power of the interrogation RF signal to DC power for supplying power to the signal processing/memory block 126. Alternatively, the RFID tag 102 may be active, carrying a discrete power source, for example a battery.
The RFID system 100 may operate in several government-regulated radio frequency bands, including a high frequency band (13.56 MHz), an ultra-high frequency band (860-915 MHz), and/or a microwave band (2.4 GHz). A conventional RFID tag 120 is tuned to a certain fixed resonant frequency. The resonant frequency depends upon the impedance of the RFID tag 120, and is a combination of the impedances of the antenna 122, the front end 124, and the signal processing/memory block 126. The resonant frequency is that frequency at which the interrogation RF signal from the RFID reader 110, for example, causes the largest electrical response in the RFID tag 120, and at which the response RF signal generated by the RFID tag 120 has the greatest range.
FIG. 2 illustrates a range of the response RF signal generated by conventional RFID tags, also referred to as the range of the RFID tag, as a function of frequency of the response RF signal. A curve labeled High Q illustrates the range of a highly tuned, high quality-factor (Q-factor) RFID tag (i.e., an RFID tag with highly tuned circuitry), and a curve labeled Low Q illustrates the range of an RFID tag with a low Q-factor. The range is maximum when the response RF signal is generated at the tag resonant frequency fR, and the range rolls-off for frequencies increasingly smaller or larger than fR. Typically, the maximum range and rate of roll-off with frequency is dependent upon the Q-factor of the tag 120.
The Federal Communications Commission (FCC) mandates that RFID systems operate in designated frequency bands. For example, if the RFID system operates in the 915 MHz band, then the RFID system is required to operate in a specified bandwidth (e.g., 30 MHz) centered about 915 MHz. Therefore, RFID tags designated to operate in the 915 MHz band are typically manufactured to have a fixed resonant frequency of 915 MHz. That is, the RFID tags will have a maximum range when generating response RF signals having a carrier frequency of 915 MHz. In addition, the RFID tags will have a maximum electrical response when queried by interrogation RF signals having a carrier frequency of 915 MHz. However, in order to allow all users access to the band with minimum interference, the FCC also mandates that an RFID system cannot operate at a single frequency for an indefinite period of time. In other words, the RFID system is required to hop to other frequencies in the specified bandwidth. For example, within a period of 4 seconds the RFID system (i.e., RFID reader 110 and RFID tag 120) may be required to operate at thirty different frequencies in a 30 MHz bandwidth. This hopping causes the range of the RFID tags to decrease as the RFID tags operate at off-resonant frequencies, thereby degrading system performance (e.g., rate of tag identification).
When an RFID system communicates at the resonant frequency of the RFID tags, such as frequency fR (FIG. 2), a population of high Q-factor RFID tags is desirable since tag identification rates will be higher than a system populated with low Q-factor tags. However, a population of tags with lower Q-factors may be desirable when the RFID system operates at some off-resonant frequencies, since the tag ranges of low Q-factor tags operating at some off-resonant frequencies are larger than high Q-factor tags operating at the same off-resonant frequencies. It would be desirable to have a system and method of maximizing tag range and tag identification, thereby increasing system efficiency, increasing tag identification rates, and decreasing system latency.